Under certain operating conditions, aircraft are vulnerable to the accumulation of contaminants on external component surfaces or skins. Examples of such contaminants include ice, water, glycol, oils and fuel. If left unchecked, the accumulation of certain contaminants, and in particular ice, can eventually so laden the aircraft with additional weight and so alter airfoil configuration as to cause undesirable flying conditions. The ability to detect the accumulation of contaminants on such surfaces, and the ability to classify those contaminants so as to identify dangerous flight conditions, has therefore become highly desirable.
A number of different kinds of contaminant detectors have been utilized for such objectives. Among these types are ultrasonic contaminant detectors, which utilize ultrasonic energy transmitted through an aircraft or airfoil skin.
One such ultrasonic ice detector is described in U.S. Pat. No. 4,461,178 issued to Jacques R. Chamuel (Chamuel). In Chamuel, an ultrasonic signal generator applies an output to a transducer which converts the signal energy into an ultrasonic wave which is passed through a portion of the airfoil skin and is sensed by a second transducer. The waveform transmitted includes a compressional portion and a flexural portion. The compressional portion and a flexural portion. The receiving transducer receives an initial waveform which corresponds to the compressional wave transmitted through the airfoil skin from the source transducer. Subsequent to the compressional wave, a larger spike corresponds to the first receipt of the flexural wave transmitted through the airfoil skin. A layer of ice on the surface of the airfoil will affect the signal waveforms, wherein the flexural wave component will be attenuated while the compressional waveform remains basically unchanged. The ratio of the peak magnitudes of the received compressional and flexural wave portions provides an indication corresponding to ice accumulation.
Control of the transmitted signal into a specific mode and waveform shape is difficult when using the normal beam excitation transducer of Chamuel, because these type of transducers have a tendency to produce higher phase velocity signals closer to cut-off frequency. Differentiating between ice and water is therefore difficult utilizing the Chamuel system. As mentioned before, it is desirable to have the ability to distinguish between ice and water because accumulated ice is dangerous for flying while accumulated water is not.
Another ultrasonic ice detector is described in U.S. Pat. No. 4,604,612 issued to Watkins, et al.. Watkins, et al. also utilizes a transmitting and receiving transducer, wherein the transmitter transmits pulses of horizontal polarized shear waves which propagate through the airfoil skin with a wavelength comparable to the thickness of the sheet so that the shear waves are guided by the surface. If the surface is dry, or covered in water (in which horizontally polarized shear waves cannot propagate) the received pulses will have a larger amplitude than if the surface is covered by a layer of ice in which shear waves can propagate. This is because some of the energy of the shear wave pulse will be dissipated due to propagation through the ice. Consequently, the Watkins, et al. ice detector can detect ice adhering to the surface but is insensitive to the presence of water.
Much like Chamuel, it is therefore impossible to detect a layer of ice covering a thin layer of water with the teachings of Watkins, et al.. Ice on water is also a dangerous condition for flying.
An ultrasonic system which can reliably identify and distinguish between the build-up of various forms of airfoil contaminants, particularly between the formation of water and ice is therefore highly desirable.